Respect -color:never in REPL output

The (successful) result of the REPL was highlighted regardless of the
value of `-color`. This commit fixes this, so that the output of the
REPL is highlighted only if `-color` is not set to `never`.

The prompt and the code as being entered are still highlighted.

Author: Duhemm

compiler/src/dotty/tools/repl/ReplDriver.scala

@@ -269,7 +269,7 @@ class ReplDriver(settings: Array[String],
         typeAliases.map("// defined alias " + _.symbol.showUser) ++
         defs.map(rendering.renderMethod) ++
         vals.map(rendering.renderVal).flatten
-      ).foreach(str => out.println(SyntaxHighlighting(str)))
+      ).foreach(str => out.println(highlight(str)))
 
       state.copy(valIndex = state.valIndex - vals.count(resAndUnit))
     }
@@ -284,7 +284,7 @@ class ReplDriver(settings: Array[String],
           x.symbol
       }
       .foreach { sym =>
-        out.println(SyntaxHighlighting("// defined " + sym.showUser))
+        out.println(highlight("// defined " + sym.showUser))
       }
 
 
@@ -339,14 +339,14 @@ class ReplDriver(settings: Array[String],
     case TypeOf(expr) =>
       compiler.typeOf(expr)(newRun(state)).fold(
         displayErrors,
-        res => out.println(SyntaxHighlighting(res))
+        res => out.println(highlight(res)(state.context))
       )
       state
 
     case DocOf(expr) =>
       compiler.docOf(expr)(newRun(state)).fold(
         displayErrors,
-        res => out.println(SyntaxHighlighting(res))
+        res => out.println(highlight(res)(state.context))
       )
       state
 
@@ -369,4 +369,11 @@ class ReplDriver(settings: Array[String],
     errs.map(renderMessage(_)(state.context)).foreach(out.println)
     state
   }
+
+  /** Add syntax highlighting to `msg` if `color` is not set to `never`. */
+  private def highlight(msg: Iterable[Char])(implicit ctx: Context): Iterable[Char] = {
+    if (ctx.settings.color.value != "never") SyntaxHighlighting(msg)
+    else msg
+  }
+
 }